Identical and Fraternal Twins: What's the Difference?

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Not Just One, But Two!

• Do you have to share a bedroom with a sibling?

Imagine sharing the same womb with your sibling from the first day you existed!

Twins are two offspring that are born from the same pregnancy.

Besides humans, this natural conception of multiple offspring happens with other mammals like horses, pigs, cattle, and armadillos. In fact, nine-banded armadillos always give birth to four identical babies, called pups!

The Case of Human Twins

Multiple births continue to rise in the United States.

From 1980 to 2018, the U.S. birthrate rose from 1.9% to 3.3% in 2018. This means that for every 1,000 pregnancies in 2018, 32.6 resulted in twins!

• What are the two types of twins?
• How do they differ from one another?

Identical Twins

Identical twins are called monozygotic twins. The prefix mono means one, and the root word zygote means a fertilized egg that will form into a new human.

This type of twin develops when one sperm fertilizes one egg. The fertilized egg then divides into two when it is still a collection of cells. Each half develops independently into new humans who have the same genetic code, hence the term identical.

• Does this mean that identical twins' fingerprints are the same?

The answer is no!

Fingerprints are not solely generated from DNA. As twins develop inside the womb, their fingers' unique features of lines and ridges develop on each baby at around 6 to 13 weeks.

Fraternal Twins

Fraternal twins are called dizygotic twins. The prefix di means two or double, and the root word zygote means a fertilized egg.

Unlike identical twins, a dizygotic development process begins when two separate eggs are fertilized by two separate sperm. Each of the fertilized eggs develops into a new human.

Fraternal twins can be both males, females, or a male and a female. The father's sperm determines the gender.

Like all other siblings, fraternal twins have half of their genetic makeup in common. They will share half of their mother's DNA and the other half of their father's. Therefore, the characteristics of fraternal twins will have crossovers or variations.

Watch the video below to learn more about identical and fraternal twins.

Factors that Affect Twinning

The occurrence of identical twins has not changed much. However, the number of fraternal twins has increased, particularly in certain countries and among certain races.

• Why might that be?
• What factors affect twinning?

Women's Age Upon Conception

In the United States, the average age at which a woman gives birth has risen steadily since the 1970s. Many women over the age of 30 now become pregnant.

Older women produce more follicle-stimulating hormone or FSH. A high FSH level increases the likelihood of producing two eggs during the menstrual cycle, one in each ovary.

Dizygotic twins are conceived if both eggs are fertilized.

Race and Heredity

African women have the highest tendency of multiple births, and Asian women have the lowest. Women who have been pregnant multiple times are also more likely to have twins.

A family history of fraternal twins on the mother's side also has an effect because it indicates a genetic cause for producing two eggs at one time. A woman, who has a twin, has twin siblings, or has previously given birth to twins, is more likely to become pregnant with twins.

Overall Health

Studies have shown that several health factors in women increase the rate of twin births. For example, taller women are more likely to conceive twins.

Women who are overweight are also more likely to conceive twins because of increased estrogen levels. An estrogen increase is also what causes women who are currently breastfeeding a child to conceive twins.

High twin rates are also found among women who consume many dairy products, possibly due to the growth hormones given to the cows.

Fertility Treatments

Fertility treatments include drugs to increase the likelihood of natural conception as well as different fertilization processes.

Women who take fertility drugs increase the number of eggs produced during ovulation, which leads to an increase in fraternal twins or even triplets! Treatments like in vitro fertilization (IVF) also increase the chance of multiple births.

Fertility treatments have also been shown to nearly double a woman's chance of having identical twins. While this is still only a 0.95% chance, that is an incredible increase.

Genetically, twins are fascinating to study and understand. As advancements in understanding conception occur, fertility treatments can occur, which will continue to increase the twin birth rate!

In the Got It? section, you will review the important ideas you have learned so far.